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July 2010
guest feature article

Coupling Rainwater Collection with Living Roofs:
A Water Quality Review
 

By David Williams and Jon Kinder, Prairie Designs LLC

901 Moreland Avenue (Eden House) in Atlanta, GA;
Photo Courtesy of David Butler.

One of the questions that frequently arises when discussing green roofs with clients is the feasibility of collecting the runoff from the roof for various uses in and around a building.

Often this interest is focused on reducing the watering requirement for the roof – after all, in drought prone areas, watering your roof seems to be wasteful, right?  If your desire is simply an ecologically friendly roof, why not just install a white roof that you do not have to water?

We will not address the green roof vs. white roof debate in this article, but we will explore the water quality implications of collecting surplus rainwater for use in watering the roof system when it needs it.  This water could also be used around the landscape, or potentially even in the building.

The latter option, using green roof runoff in buildings as potable water, almost always raises eyebrows.  Would you want to be drinking water that just passed through soil?

We have heard differing opinions on this topic, and were very interested when we found a recent report from the Texas Water Development Board entitled: Effect of Roof Material on Water Quality for Rainwater Harvesting Systems (January 31, 2010).  This report is available for free, and has some applicable findings to our field.

As the title mentions, the study centered on the effect that different roof treatments have on the water quality of collected rainwater.  A survey of the most common roofing materials in Texas was completed, and found that the three most common were asphalt-fiberglass shingles, Galvalume® metal panels, and concrete tiles.

 The Lady Bird Johnson (LBJ) Wildflower Center Test Roofs;
Graphic Source: Texas Water Development Board,
Effect of Roof Material on Water Quality for Rainwater Harvesting Systems

Three 8 foot x 4 foot test roofs were constructed at the Lady Bird Johnson (LBJ) Wildflower Center in Austin to test these three roofing treatments. Some readers may already know that the LBJ Wildflower Center is home to one of the first green roof research plots in the state (Publisher's Note:  See the profile in The Greenroof & Greenwall Projects Database here), testing various commercial green roofing systems and other cool roof technologies over the course of many a hot, Texas summer.

  The LBJ Test Greenroof Plots; Photo Courtesy of Steve Windhager, PhD.

In addition to monitoring runoff from these three new test roofs, the team also decided to monitor runoff from one of the green roofing modules on site, as well as a white roof from the same study.

The team followed best practices for collecting rainwater, fitting all collectors with first flush systems to capture and sequester the dirtiest water before allowing additional water to flow into the two subsequent storage tanks.  Such first flush systems are a standard feature on most rainwater harvesting systems because the initial surge of rainfall across the roof washes any contaminants on the roof into the collection system.

Southern Illinois University that found elevated levels of Lead and Cadmium (Cd) leaching sporadically from many common green roof soils (Alsup et al. 2010).

The collected water from each treatment was tested for a number of water quality indicators, which are discussed below.

Results

Conductivity

 Graphic Source: Texas Water Development Board,
Effect of Roof Material on Water Quality for Rainwater Harvesting Systems

Conductivity is the ability of a substance to conduct an electric current, and it turns out that extremely pure water is less conductive than water with dissolved salts or other impurities floating around. This is because these impurities tend to ionize while in solution, and ions (non-neutrally charged particles) allow electric current to flow more freely through a medium.  The green roof used in this study produced the most conductive water samples.

It is hard to draw a finite conclusion from this, except to say that the collected green roof runoff had more impurities in it.  Impurities are not necessarily good or bad; highly conductive water often simply has a more mineral taste to it.  If the Wildflower Center Staff had been using city or well water from the Austin area to water this roof, it is possible that minerals from that water were deposited into the green roof soil and have been subsequently leeching out into the runoff.

Turbidity / TSS

Turbidity is one measure of how many suspended solids are floating around in a fluid.  These solids, in high enough concentrations, will cause a clear fluid like water to appear cloudy or opaque.  One might expect that water passing through soil would be full of suspended solids; shouldn’t some of that dirt be washing out?

It turns out that the green roof runoff had the lowest turbidity out of any roofing treatment tested:


“The lowest turbidity values were found in rainwater harvested after the first flush from the green roof…an indication that green roofs can effectively filter out particles. It is important to note, however, that all roofs yielded higher turbidity values than the 1 NTU maximum recommended for potable use of harvested rainwater, which is the same as the U.S. EPA’s guideline for filtered surface water.”


The results were similar for Total Suspended Solids (the measure of how much stuff collects on a filter when a sample of water is poured through it); the green roof showed the lowest values.

Take home point: if you’re going to drink your collected rainwater, you should first run it through a sediment filter to remove suspended solids.

Nitrates

Nitrates in water come from a number of sources, and have potential health implications especially for very young children. While health effects in adults are mild and rare, the presence of nitrates in water usually indicates anthropogenic contamination of some kind in the water.

Because the water samples in this study were collected from roof surfaces, they are less susceptible to the more common vectors of nitrate contamination (fertilizer, human/animal waste).  Almost all of the samples were below the EPA limit, and the green roof had the lowest values of all treatments tested.

Dissolved Organic Carbon (DOC)

DOC tests measure the amount of organic material in a solution.  Organic materials from plants and animals can break down to such small sizes that they can become dissolved in water.  ecause DOC is derived from living organisms, it makes sense that the water collected from the green roof had the highest levels of DOC’s.

The big implication here is on the use of chlorine to disinfect collected rain water.  Chlorine reacts with DOC to form all sorts of not-so-good-for-you byproducts.  To avoid this problem all together, collected rainwater from a green roof should be treated with a disinfectant other than chlorine before use.  UV light “filters” and ozone bubblers are proven alternatives to Chlorine.

Coliform

 Graphic Source: Texas Water Development Board,
Effect of Roof Material on Water Quality for Rainwater Harvesting Systems

Runoff from the green roof was lowest in both Total Coliform and Fecal Coliform compared to the other treatments, but both were present in all treatments after the first flush.  Coliform can be eliminated with a UV or ozone disinfectant system, but a green roof goes a long way towards eliminating it before filtration.

Heavy Metals

 Graphic Source: Texas Water Development Board,
Effect of Roof Material on Water Quality for Rainwater Harvesting Systems

Roof runoff was also tested for Aluminum (Al), Iron (Fe), Copper (Cu), Zinc (Zn), Lead (Pb), and Chromium (Cr).


“For all rain events, rainwater harvested after the first flush from the green roof consistently showed the lowest concentrations of Al, Fe, Cr, and Cu…the highest Zn concentrations were seen in the harvested rainwater after the first flush from the green and metal roofs; elevated Zn concentrations from the green roof might have been from the solder in the scupper gutter.”


Additionally, the solder in the gutter was also a potential source for the Lead found in the green roof runoff during one rain event.

These are great results, but further study is needed to determine if elevated Zinc and Lead levels were due to contamination outside of the roof or not.  If the gutters are not to blame, this may corroborate a study conducted at Southern Illinois University that found elevated levels of Lead and Cadmium (Cd) leaching sporadically from many common green roof soils (Alsup et al. 2010).

This question highlights the need for further data on manufactured green roof soils and their propensity to leach heavy metals, specifically Lead and Cadmium.  Additionally, it is always smart to consider what materials your potentially-potable harvested rainwater flows across or through on its way to your tanks; many roofing/guttering materials contain these heavy metals.

Considerations

While this study is very informative, it only analyzed a small number of rain events on small test roofs.  Furthermore, the green roof and white roof were almost flat, while the other three treatments were inclined at just over 18 degrees.

The team also found extreme variability when their analysis were scaled up to full sized residences located in different areas of the city, suggesting that geographical location plays a role in determining what ends up in your collection tanks.
Regardless of the source of your rainwater, you should always follow the best practices when designing and installing your system to prevent contamination.  In association with others, the Texas Water Development Board has produced "The Texas Manual on Rainwater Harvesting," a comprehensive manual that is available free, and considered by many to be one of the best resources on the subject.

Conclusion

So how does this study affect our thoughts about harvesting rainwater from a green roof?

It would seem that most harvested rainwater can easily be made safe to drink if the levels of heavy metals are not too high.  In many cases, runoff from a green roof enters the filtering/disinfecting process cleaner than water harvested from many popular roofing alternatives – especially with respect to heavy metals.

One should keep in mind that the volume of rainwater collected from a green roof will be less than the volume collected from a less absorbent roofing material.  Many studies suggest that around 60%-80% of the raindrops that fall on a green roof will never make it into a collection tank, thus reducing the roof’s effectiveness at producing storable water.  This fact can be both positive and negative, depending on project priorities.

Because less water is captured per square-foot from a green roof vs. a metal or shingle roof, green roofs can use smaller, cheaper water storage systems.  In addition, if stormwater management is a primary project goal, combining rainwater collection with living roofs can be a great tactic.  Doing so will effectively increase the roof’s capture efficiency to nearly 100% while providing a sustainable source of irrigation water for the roof during the summer months.  It is worth noting that deciding against a green roof may be the right call if a project or client needs larger quantities of captured water; it is especially important that consultants be well versed in both technologies to help a client make the right decision.

While using collected green roof rainwater for potable purposes still may be a few years away, the benefits of collecting green roof runoff for landscape watering applications have inspired multiple green roof projects to implement rainwater harvesting systems.  Here are a few:

• The high-profile Beddington Zero Emission Development (BedZed) – Beddington, UK
Circular Congregational Church - Charleston, SC
251 East Medical Center - Houston, TX
The Solaire - 20 River Terrace - New York, NY
Southface Eco Office - Atlanta, GA
901 Moreland Avenue (Eden House) - Atlanta, GA


David Williams and Jon Kinder


Further Reading

The Texas Manual on Rainwater Harvesting

Texas A&M Online Rainwater Resources

Rainwater Harvesting for Drylands and Beyond, Volume 1

References:
Alsup, S., Ebbs, S., & Retzlaff, W. (2010). The exchangeability and leachability of metals from select green roof growth substrates. Urban Ecosystems, 13(1), 91-111. doi:10.1007/s11252-009-0106-y

Mendez, C. B., Afshar, B. R., Kinney, K., Barrett, M. E., & Kirisits, M. J. (2010). Effect of Roof Material on Water Quality for Rainwater Harvesting Systems. Texas Water Development Board: P.O. Box 13231, Capitol Station Austin, Texas 78711-3231.

The Texas Manual on Rainwater Harvesting, Third Edition. (2005). Retrieved from https://www.twdb.state.tx.us/RWPG/rpgm_rpts/2003483510_RainwaterHarvesting.pdf
 

Dave Williams, LEED AP and Jon Kinder, LEED AP formed Prairie Designs in 2009 after completing green roof research at Texas Christian University in Fort Worth. Their research on adapting green roofs for Texas was integral in the development of the living roof on the new headquarters of the Botanical Research Institute of Texas.

Prairie Designs' primary focus is creating drought-tolerant living roofs based on native ecosystems. Additionally, they manufacture Roof Rocks™, a patent-pending mulching system that drastically reduces moisture loss in green roofing systems.

To learn more about Prairie Designs, please contact Dave Williams at dave@prairiedesignsllc.com and Jon Kinder at jon@prairiedesignsllc.com or call: 512.827.ROOF (7663).

 

Past Guest Feature Articles

The opinions expressed by our Guest Feature writers and editors may not necessarily reflect the beliefs of Greenroofs.com, and are offered to our readers to simply present individual views and experiences and open a dialogue of further discussion, debate and research.  Enjoy, and if you have a particular comment, please contact the author or send us an email to:  comments@greenroofs.com.


 

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